1. Field of the Invention
The present invention relates to an analog input signal processing circuit for inputting to a microprocessor signals related to signal voltages of a variable analog signal source and, more particularly, to an improved analog input signal processing circuit for making the variable control of gain of an input circuit and frequency characteristics with high accuracy.
2. Description of the Related Art
An analog input signal processing circuit having both functions to adjust gain and to adjust frequency characteristics is well known.
For example, according to the Japanese Patent Publication (unexamined) No, 16460/2002 “Gain Control Circuit”, proposed is a concept in which the adjustment of gain is made by changing a switching duty ratio of switching elements that are connected in parallel or in series with a resistor to determine a gain of an operational amplifier; and the control of frequency characteristics of a filter is made in an AC amplifier.
Further, a switched capacitor filter has been widely used as a component of a filter circuit.
For example, according to the Japanese Patent Publication (unexamined) No. 205113/1999 “Switching Circuit and Switched Capacitor Filter”, proposed is a concept in which an equivalent variable resistance, of which resistance value R is expressed with R=Ts/C1, is obtained by charging and discharging a capacitor of a capacity C1 in a variable period Ts.
According to the Japanese Patent Publication (unexamined) No. 130043/2002 “Signal Processing Unit”, proposed is a concept of a knock determination device of an engine using a switched capacitor filter circuit acting as a band pass filter, a variable gain amplifier circuit, and a peak hold circuit.
Further, according to the Japanese Patent Publication (unexamined) No. 306645/1993 “Internal Combustion Engine Knocking detection Device”, proposed is a concept of adjusting signal pass frequency bands of a switched capacitor filter forming a bandpass filter (hereinafter simply referred to as “band filter”) depending on operation state of an internal combustion engine.
In the gain control circuit according to the above-mentioned Japanese Patent Publication (unexamined) No, 16460/2002, no concept of changing a switching frequency of switching elements is included, and gain and frequency characteristics of a filter are changed in cooperation by changing a switching duty ratio of switching elements, thus being incapable of changing and adjusting individually the maximum gain and frequency characteristics.
Moreover, in the signal processing circuit according to the above-mentioned Japanese Patent Publication No. 130043/2002 or the above-mentioned Japanese Patent Publication No. 306645/1993, signals of adjusting filter characteristics and signals of adjusting gain characteristics are separated respectively, so that it is necessary for a control section to provide two kinds of control signals.